1. Field of the Invention
The present invention relates to an optical system and an optical apparatus including the same, for example, an optical apparatus such as a silver film camera, a digital still camera, a video camera, a telescope, a binocular, a projector, or a copying machine.
2. Description of the Related Art
The optical system used for the optical apparatus such as a digital camera or a video camera has to be short in total lens length (total optical system length: length from first lens surface of object side to image surface), and small in overall size.
Generally, as the optical system is smaller, more aberrations, in particular chromatic aberrations such as on-axis chromatic aberrations or magnification chromatic aberrations occur, to reduce optical performance.
In the optical system of a telephoto type (telephotographic) where a total lens length is shortened, in particular, more chromatic aberrations occur as a focal length is extended more (lengthened more).
As a method for reducing such chromatic aberrations, an optical system has been known, which performs achromatization (achromatic correction) by using an abnormal partial dispersed material for a lens material.
In the optical system of the telephoto type, to correct chromatic aberrations, a positive lens made of a low dispersed material such as a fluorite having abnormal partial dispersion for a lens group of the optical system front side where heights of a paraxial marginal ray and a paraxial chief ray from an optical axis are both high can be used.
An optical system of a telephoto type has been known, which corrects chromatic aberrations by using a material having abnormal partial dispersion (Published Japanese Patent No. 60-49883 and Japanese Patent Laid-Open No. 11-119092).
Further, an optical system has been known, which performs achromatization by using, as a solid-state material having abnormal partial dispersion characteristics, a solid-state material made of a mixture containing dispersed indium-tin oxide (ITO) fine particles for a transparent medium (Japanese Patent Laid-Open Nos. 2005-181391 and 2005-215387).
Further, an optical system which performs achromatization by using, as a solid-state material having abnormal partial dispersion characteristics, a solid-state material made of a mixture containing dispersed TiO2 fine particles for a transparent medium has been known (See Japanese Patent Laid-Open Nos. 2006-14582 and 2006-3499948).
On the other hand, focusing in an image-taking optical system (an optical system) used for an image-taking apparatus such as a camera is performed by using a method which moves the entire image-taking optical system or a partial lens group of the image-taking optical system in many cases.
An image-taking optical system of a telephoto type having a long focal length is generally large in size, and heavy in weight. Thus, it is mechanically difficult to perform focusing by using the entire image-taking optical system. As a result, many image-taking optical systems of telephoto types use an inner-focusing system which performs focusing by using a relatively small and light lens group other than a lens group of the image-taking optical system closest to an object.
An image-taking optical system has been known, which includes three lens groups: sequentially from an object side to an image side, a first lens group of a positive refractive index, a second lens group of a negative refractive index, and a third lens group of a positive refractive index, and performs focusing by moving the second lens group on an optical axis (See Japanese Patent Laid-Open No. 11-316341).
An optical system of an inner-focusing type has been additionally known, which includes, sequentially from an object side to an image side, first to fifth lens groups of positive, negative, positive, negative, and positive refractive indices, and performs focusing by moving the second lens group on an optical axis (See Japanese Patent Laid-Open No. 2000-89101).
In the optical system of Japanese Patent Laid-Open No. 2000-89101, the fourth lens group is moved in a direction perpendicular to the optical axis to correct image blurring generated when the optical system vibrates, in other words, to perform vibration prevention.
In the optical system of the telephoto type, chromatic aberrations can be remedied while shortening a total lens length, for example, by using a lens made of a fluorite having abnormal partial dispersion characteristics and increasing a refractive power of the lens. However, in a lens made of a low-dispersed glass such as a fluorite having a large Abbe number, when a refractive power is increased to correct chromatic aberrations, aberrations other than the chromatic aberrations, such as spherical aberrations, coma aberrations, or astigmatism occur by a large number.
Number of lenses having abnormal partial dispersion characteristics has to be increased in order to correct these aberrations concurrently with the chromatic aberrations. However, the increased number of lenses leads to enlargement of the entire optical system and an increase in weight.
A glass material such as a fluorite having abnormal partial dispersion characteristics can not be repeatedly used because the processing is extremely difficult and a surface can be easily damaged.
As materials which indicates abnormal partial dispersion characteristics, solid-state materials such as TiO2, ITO, and a UV-curing resin disclosed in Japanese Patent Laid-Open Nos. 2005-215387, 2006-145823, and 2006-349948 can be used in order to achromatize the optical system.
When a refractive optical element made of such an effective solid-state material having abnormal partial dispersion characteristics is used in the optical system, it is important to appropriately set a position in an optical path, a refractive power or the like.
Inappropriate setting of such conditions leads to a difficulty of obtaining high optical performance by miniaturizing the entire optical system and correcting chromatic aberrations well.